1. Field of the Invention
This invention relates to an optical tomography system for obtaining an optical tomographic image by measurement of OCT (optical coherence tomography).
2. Description of the Related Art
When obtaining tomographic image of an object of measurement in a body cavity, there has been sometimes used an optical tomography system employing the measurement of OCT. In the optical tomography system, low coherence light emitted from a light source is divided into measuring light and reference light and the measuring light is projected onto the object of measurement, while the reflected light from the object of measurement is multiplexed with the reference light, and a tomographic image is obtained on the basis of the intensity of the interference light of the reflected light and the reference light.
In the optical tomography system, there has been known a system using the TD-OCT (time domain OCT) measurement where the measuring position in the direction of the object is changed by changing the optical path length of the reference light.
Further, recently, as a system of rapidly obtaining a tomographic image without changing the optical path length of the reference light, there has been proposed an SD-OCT system using an SD-OCT (spectral domain OCT) measurement. In the SD-OCT measurement, a tomographic image is formed without scanning in the direction of depth, by dividing broadband, low coherence light from a light source by the use of a Michelson interferometer, and carrying out a Fourier analysis on each channeled spectrum obtained by decomposing the interference light of the reflected light, which returns when projecting the measuring light onto the object, and the reference light into frequency components.
In the SD-OCT system, frequency analysis is carried out on the interference light by the use of a spectrometer which spectrally divides the interference light and a detector which detects by the frequencies the intensities of the light spectrally divided by the spectrometer. In Japanese Unexamined Patent Publication No. 2000-046729, there is disclosed an SD-OCT system in which a diffractive optics is employed as the spectrometer.
However, since the diffractive optics exhibits polarization characteristics, the output of the spectrometer differs depending on the state of polarization of light impinging upon the spectrometer. For example, as shown in FIG. 9, light where the direction of vibration of the electric field vector is parallel to the groove of the diffractive optics (S-polarization: shown in broken line) differs in diffracting efficiency from light where the direction of vibration of the electric field vector is perpendicular to the groove of the diffractive optics (P-polarization: shown in solid line). In FIG. 9, the abscissa represents λ/d which is the ratio of the pitches d of the lattices in the diffractive optics and the wavelength of the incident light and the ordinate represents the diffracting efficiency when the blaze angle θB is 30°. As shown in FIG. 9, when λ/d=1, the diffracting efficiency is almost l in the case of the S-polarization, whereas the same is only about 0.6 in the case of the P-polarization. That is, in the case where the interference light is linearly polarized, there generates a difference close to about twice in the diffracting efficiency depending upon the direction of the polarization of the interference light. Since the difference in the diffracting efficiency appears as a difference in the intensity, the wavelength spectrum output from the spectrometer fluctuates according to the state of polarization.
In an optical tomography system applied to an endoscope, a fiber is employed to guide light to a body cavity. The fiber is inherently folded or twisted since it is inserted into a body cavity. Further, a temperature change of the fiber inherent to insertion of a body cavity is necessarily generated. In a single-mode fiber which is generally used for an endoscope, polarization of light propagating through the fiber changes due to the stress by folding or twisting or a temperature change. Further, in an endoscope, light is often caused to be polarized and to make one rotation in a circumferential direction of the fiber. When the light makes one rotation, the direction of polarization is rotated by 360°. Due to these facts, the state of polarization of the interference light guided through the fiber during measurement is not constant but fluctuates.
When the fluctuating interference light impinges upon the spectrometer, the wavelength spectrum changes and the change is considered to be error in the case of the spectrometers exhibiting polarization characteristics. The level of a signal obtained by Fourier-transform of the wavelength spectrum also fluctuates and as a result, a tomographic image deteriorates in image quality.
Further, in the SD-OCT system, a broad-band light source is necessary to ensure a high resolution, which involves the following problem due to the polarization characteristics. For example, when a light source which is 1 μm in central wavelength, and 200 nm in wavelength width is used, and a diffractive optics which is 1 μm in pitches d of the lattices is used as the spectrometer, λ/d is 0.9 to 1.1 since the wavelength band of the light source is 900 to 1100 nm. In this case, in accordance with FIG. 9, the diffracting efficiency is almost unchanged in the case of the S-polarization, whereas the same is 0.5 to 0.7 according to the wavelength in the case of the P-polarization.